The prior art has disclosed hydrostatic transmissions for mobile working machines in which a hydrostatic pump (primary unit) and a hydrostatic motor (secondary unit) are fluidically connected to one another by means of a closed circuit. An internal combustion engine, for example a diesel engine of the mobile working machine, is coupled rotationally conjointly to the primary unit, and an output, for example an axle or a wheel of the mobile working machine, is coupled rotationally conjointly to the secondary unit. The mobile working machine thus has a traction drive which has a hydrostatic transmission.
The document EP 1 960 699 B1 discloses a hydrostatic transmission of said type, with which braking can also be performed. Here, in relation to traction operation, the power flows in the reversed direction from the output, via the secondary unit acting as a pump and via the primary unit acting as a motor, to the internal combustion engine, which is then driven in a passive cranking operating mode. The highly pressurized working line of the closed circuit is in this case safeguarded by means of a pressure-limiting valve, by means of which, too, a part of the braking power can be dissipated during the braking operation.
A problem during such braking operations is that the internal combustion engine may be driven only with a maximum rotational speed in order that it is not destroyed. Therefore, the primary unit, which is designed as an adjustable axial piston machine and which operates as a motor is, during the braking operation, permanently set to a pivot angle at which a torque is generated that can still be supported by the internal combustion engine without the latter being caused to overspeed. This occurs taking into consideration the pressure determined in the working line by the setting of the pressure-limiting valve.
A disadvantage of the hydrostatic transmissions of the prior art is that, during the braking function, the internal combustion engine is not brought to its maximum rotational speed. It is therefore disadvantageously necessary for a considerable part of the braking power to be dissipated by means of the pressure-limiting valve of the high-pressure line in question.
The documents DE 10 2014 211 393 A1 and DE 10 2014 211 394 A1 each likewise disclose a hydrostatic transmission with which braking can also be performed, wherein it is sought to protect the internal combustion engine against overspeeding. A first part of the braking power is output via the primary unit to the internal combustion engine, whereas a second part of the braking power is converted by means of the pressure-limiting valve of the high-pressure line in question into heat. Here, a rotational speed of the internal combustion engine which is initially slightly higher than the admissible rotational speed limit is accepted.